Phyte

An enigmatic, caustic, and extremely deadly fog-like substance which is responsible for the end of the world. It drifts silently, dreamily across the landscape, annihilating civilization without immediately and without fanfare. No one knows where it came from, but most of the world is lost. The Creadores, Cultivo's gods themselves took mortal form to combat it.
Upon contact with the fog, animals and plants are changed irreversibly, becoming the hostile, strangely proportioned Roughers and the violently prolific Toughers respectively. People simply die. Espuna dissolve, Bakdreskr dessicate, Suevira rupture, shattering into pieces.
The Creadores have found that their divine power can destroy the fog, but more always takes its place. It is inexorable, always pushing against the wards of the last remaining bastion of civilization, Orden. It is the tide, patiently breaking down the levees of the gods wards, ready to push itself through even the smallest crack and obliterating the last people of Cultivo.

Manifestation

The Phyte is a black, billowing fog. It is lightly opaque and drifts low to the ground. It blows in the wind and floats on top of water. It obeys general fluid dynamics, except that it can spread up hills and mountains. It has no sound, and its scent is light, distinct, and indescribable.